Generally, a compression-type refrigerator includes a compressor, a condenser, an expansion mechanism (e.g., expansion valves), an evaporator, and an optional drying apparatus, and a liquid mixture of refrigerant and lubricating oil (refrigerator oil) circulates in a closed system of the refrigerator. In such a refrigerator, generally, the compressor is operated at high temperature, while the cooler is operated at low temperature, although the difference in temperature varies depending on the type of the refrigerator. Therefore, a refrigerant and a lubricating oil must circulate in the system without causing phase separation within a wide temperature range of high to low temperature. Generally, a mixture of refrigerant and lubricating oil has a phase separating region in a low-temperature range and in a high-temperature range. The highest phase separation temperature in the low-temperature range is preferably −10° C. or lower, particularly preferably −20° C. or lower, whereas the lowest phase separation temperature in the high-temperature range is preferably 30° C. or higher, particularly preferably 40° C. or higher. When phase separation occurs during operation of a refrigerator, the service life and efficiency of the refrigerator are considerably impaired. For example, when phase separation between refrigerant and lubricating oil occurs in a compressor, lubrication of a movable member is impaired, thereby causing seizure or a similar phenomenon, resulting in considerable shortening of the service life of the refrigerator. When phase separation occurs in an evaporator, a viscous lubricant remains, thereby lowering heat exchange efficiency.
Hitherto, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), etc. have been often employed as refrigerants for refrigerators. However, since these compounds contain chlorine, causing an environmental problem, use of chlorine-free alternative refrigerants such as hydrofluorocarbons (HFCs) has been investigated. However, since HFCs are substances that possibly contribute to global warming, natural refrigerants such as hydrocarbon, ammonia, and carbon dioxide have become of interest for use as environmental friendly refrigerants.
Meanwhile, lubrication performance of a lubricating oil for refrigerators, which is employed for lubricating a movable member of a refrigerator, is an important property. Particularly when a lubricating oil works in a compressor at high temperature, the lubricating oil is required to exhibit such a viscosity that the oil provides a stable oil layer that attains sufficient lubrication. Generally, the required viscosity varies depending on the type of the employed compressor and operational conditions. Before being mixed with a refrigerant, the lubricating oil preferably has a kinematic viscosity of 10 to 200 mm2/s at 40° C. It has also been accepted in the art that, when the viscosity is lower than this range, the oil layer becomes thin to readily cause lubrication failure, whereas when it is higher than the mentioned range, heat exchange efficiency is impaired.
There is disclosed a lubricating oil composition for use in a vapor-compressing refrigerating machine employing carbonate gas as a refrigerant, wherein the composition contains a lube base oil as a predominant ingredient, the lube base oil exhibiting a 10%-recovery temperature of 400° C. or higher and a 80%-recovery temperature of 600° C. or lower, as determined through simulated distillation by gas chromatography; a kinematic viscosity (at 100° C.) of 2 to 30 mm2/s; and a viscosity index of 100 or higher (see, for example, Patent Document 1).
The kinematic viscosity of the base oil contained in the lubrication oil composition is 17 to 70 mm2/s as determined at 40° C. (see Examples in Patent Document 1).
When a refrigerator oil having such a high viscosity is employed in a refrigerator, energy consumption of the refrigerator unavoidably increases. Thus, lowering viscosity of a refrigerator oil and improving friction characteristics during lubrication have been studied, for the purpose of saving energy of refrigerators.
Taking a refrigerating machine for refrigerators as an example, the viscosity of the lubricating oil was previously lowered to VG32, 22, 15, or 10, whereby energy saving effect was improved. However, further lowering the viscosity poses problems such as lowering sealing performance and lubricity.
Patent Document 1:
Japanese Patent Application Laid-Open (kokai) No. 2001-294886